When a display device where the number of displayable gradations is small performs multiple gradation display, smooth variation of brightness cannot be displayed. The brightness varies in steps, a contour-like pattern appears, and the image quality is reduced.
For example, a display device such as a plasma display panel (PDP) or a digital mirror device (DMD) displays multiple gradations by dividing one field to a plurality of subfields having different luminance weights and combining light emission and non-emission in each subfield. It is known that specific noise, known as a motion picture pseudo-outline, occurs in a motion picture part in this display device. For preventing the occurrence, the image display may be performed using not the gradations where the motion picture pseudo-outline is apt to occur but the gradations (hereinafter referred to as “available gradations”) where the motion picture pseudo-outline hardly occurs. Here, an error diffusing method can be used for artificially displaying all gradations using available gradations. For example, Japanese Patent Unexamined Publication No. 2000-276100 discloses a method having a gradation converting means for converting a gradation to be displayed to an available gradation. In this method, when the gradation to be displayed is not available, this gradation is converted to an available gradation closest to itself, and the error occurring at this time is diffused to surrounding pixels and summed up. Thus, the gradation to be displayed is artificially displayed using only the available gradation.
It is advantageous from the aspect of the cost and mounting that a memory element such as a static random access memory (SRAM) is used as a gradation converting means. However, the access time of the SRAM is relatively slow, and an error cannot be diffused to the immediate right pixel when the access time exceeds one-data period. The SRAM cannot therefore be employed as the gradation converting means. Here, the one-data period is about 41 ns when one frame is 852×480 pixels, for example.
For addressing this problem, Japanese Patent Unexamined Publication No. 2000-276100 also discloses a method of using a lower bit of an image signal as a temporary error and diffusing the error to the immediate right pixel.
However, the error diffused to the immediate right pixel is not an accurate value in this method, so that the image quality is disadvantageously reduced compared with a case of correct error diffusion processing.
The present invention addresses the problems discussed above. The present invention provides an image signal processing device and an image signal processing method that can perform error diffusion processing to accurately diffuse an error to the immediate right pixel even when the access time of a means for converting the gradation exceeds one-data period.